Grain size, composition, porosity and permeability contrasts within cross-beddedudsandstones in Tertiary fluvial deposits, central Spain

摘要

Permeability measured with a portable probe pcrmcflmeter on outcrops of cross·bcdded sandstones rangesudbetwecn 0·9 and 19 O. The highest pcrmeability (2-19 0 with an avcrage of8·5 D) occurs in thc coarsestudgrained foresct laminae (CFL). intermediate values (2-120 with an average of 5·3 D) occur in fincrudgrained foreset laminac (FFL) and thc lowest values (0·9-1 0 0 with un avcrage of 4·8 D) occur in boltomsctudlaycrs (BL). In the cross·bcds the uverage grain sizc rangcs from medium graincd sand in the CFL to fincudgrained s,lnd in thc FFL and BL. In all three subf'lcies. the average size of the primary pores isudapproximately It/> unit smaller than the averagc grain size. The abundance of unstablc carbonatc clastsudcorrelatcs with incrcasing avcrage grain size, micritic clasts being most abundant in the CFL. Converscly.udquartz content incrcascs with decreasing grain size and is highest in the FFL and BL. Oiageneticuddestruction of primary porosity by compact ion and cementation. as well �IS generation of sccondary porosityudthrough dissolution, were controlled by the original mineralogical composition of the sand. Contrasts inudgrain size detcrmine the primary pore size contrasts and differences in composition bctwecn CFL. FFLudand BL. Permeability contrasts reflect variations in averagc primary pore size rather than differenccs inudtotal porosity. Probe permeability contrasts between eFL. FFL and BL depend on contrasts in llverageudpore size and contrasts in mineralogical composition between the subfacies.